The present invention relates to a matrix infrared detector comprising a matrix of infrared elements, particularly for a camera, for example for aerial reconnaissance.
The advantage of such matrix detectors lies in the fact that addressing, or scanning, of the elements of the matrix is effected electronically, and no longer mechanically, by a luminous spot.
Addressing consists, in fact, with the aid of electronic processing means, either of integrating the signals delivered by the elements of the matrix, or of sequentially commuting the elements of the matrix on a circuit for reading the signals delivered. It will therefore be readily appreciated that each of the elements of the matrix is connected to these electronic processing means.
To obtain the optimal conditions of operation and sensitivity, the matrix of infrared elements must operate at low temperature.
Thus, for spectral bands of 3 to 5 .mu.m or of 8 to 12 .mu.m, for example, the optimal temperature is 77.degree. K. for the detector made of Hg Cd Te or In Sb.
It has therefore been proposed to dispose the matrix of elements on a cooled surface, in a cryogenic enclosure.
It was firstly proposed to couple the elements of the matrix to electronic processing circuits, outside the cryogenic enclosure, by connections. However, this solution presents drawbacks due to the length of the connection lines, in which parasitic noises are created, and to the number of such lines, which presents a thermal load due to line losses. Furthermore, this solution leads to detection modules of large dimensions, with a large number of elementary solders.
It was then proposed to dispose the matrix of infrared elements and the electronic circuits on the same cold surface, inside the cryogenic circuit.
However, the dimensions of this surface are necessarily limited by the power of the coolers. This latter solution therefore led to adopting, as electronic processing circuits, integrated circuits of silicon, of which mastery was acquired, making it possible to gather together on a small surface a large number of electric functions, with photovoltaic, and not photoconductor, infrared elements, for reasons of impedance matching, with a very large number of sensivive elements.
Two solutions were then possible: Either dispose the matrix of infrared elements and the silicon circuit one beside the other, or dispose the matrix above the silicon circuit.
The first of these solutions did not offer a decisive advantage either from the point of view of dimensions or from the manufacturing standpoint.
By adopting the second solution, it was still possible to opt either for a technology in which illumination of the matrix by the radiations is effected by the face bearing the detector diodes, i.e. the front face remote from the silicon circuit, or for a technology with illumination by the rear face, opposite the one bearing the diodes.
In rear face illumination technology, the difficulty of making a monocrystalline, for example Hg Cd Te, detector layer on a substrate transparent to the radiations is encountered. In this case, as the sensitive elements are adjacent the silicon circuit, the connections by cold welding between the studs for connecting the silicon circuit and the detector layer, for example of indium, may, of course, easily be effected.
In front face illumination technology, orifices are made in the monolithic active wafer to produce, by thin layers, links between the zones of the wafer with doping different from that of the rest of the wafer and defining the diodes of the wafer, and the input studs of the silicon circuit, to the rear of the wafer, thus creating islands of detector material relatively offset with respect to one another to allow connections on the silicon circuit. In this case, the ratio between the surface of the active portion and the total surface of the wafer, i.e. the coefficient of occupation, is low. In addition, this island structure is very fragile. Finally, and as in the rear face illumination technology, it is difficult to make a large number of connections.
It is an object of the invention to overcome the drawbacks of this front face illumination technology.